Polymer grade isoprene is obtained by purification of crude isoprene. In a petrochemical plant, crude isoprene is produced mainly as a coproduct in the production of ethylene by the cracking of naphtha, gas oil or crude oil, usually by selectively recovering a predominantly C.sub.5 stream. Crude isoprene, so produced, contains acetylenic contaminants such as isopropenylacetylene (hereafter "IPEA" for brevity) and butyne-2 (hereafter "BY-2" for brevity), other pentenynes and pentadienes such as cyclopentadiene, (hereafter "CPD" for brevity), the presence of any of which, in even minute quantities, is deleterious to the polymerization of isoprene in the presence of a polymerization catalyst. Prior art processes were directed to the removal of as much of these contaminants as possible. Thus, whether the prior art process was based on hydrogenation, oxidation, hydration, adsorption or cracking, the process sought to remove acetylenic (alkyne) impurities and CPD, which individually and severally were poisons for the polymerization catalyst. Since CPD is a diolefin, a process directed to its removal also removed a significant, and economically onerous fraction of desirable diolefins. Many of the prior art processes, the most relevant of which will presently be referred to, utilize certain Group I B catalysts, and particularly cooper metal, or oxides of copper either individually or in combination with activators (promoters), over a wide range of operating conditions. Each of the prior art processes failed to recognize that the key to a successful process was to allow CPD to go unconverted, if it was present in the feed in any amount sufficient to be deleterious to the polymerization catalyst, and to remove unconverted CPD in a subsequent purification step.
One of the earliest attempts to catalytically purify conjugated diolefins contaminated with acetylenic hydrocarbons is documented in U.S. Pat. No. 2,398,301 (4/1946) to Frevel, L. K. Soon thereafter, another catalytic process, stated to be a selective hydration process, was disclosed in U.S. Pat. No. 2,408,970 to Doumani et al for the removal of acetylenic impurities in hydrocarbon mixtures containing butadiene. At present, minor quantities of IPEA, and other C.sub.2 -C.sub.5 acetylenes are removed by selective hydrogenation of the acetylenes over a supported copper catalyst. Such processes are taught in U.S. Pat. Nos. 3,076,858 to Frevel et al; in 3,634,536 to Frevel L. K. and Dressley, L. J.; and, 3,751,508 to Fujiso et al, inter alia. Not long thereafter, an adsorption process carried out in the temperature range of 25.degree. C.-175.degree. C. was disclosed in U.S. Pat. No. 3,754,050 to Duyverman et al. Still more recently, U.S. Pat. No. 3,897,511 to Frevel, L. K. and Dressley, L. J., disclosed a catalytic process for removal of alpha-acetylenic impurities by their adsorption on a supported catalyst consisting essentially of a mixture of finely divided copper metal and a minor proportion of at least one polyvalent activator metal.
From a practical point of view, it is economically undesirable to hydrogenate a large feedstream of crude isoprene, no matter how selectively the hydrogenation can be effected. Until the discovery of the process of this invention the only realistic option for adequate removal of IPEA from a crude isoprene stream was by distillation wherein IPEA was removed overhead with a sacrificially large percentage of isoprene. The isoprene-rich bottoms stream containing less than 60 ppm IPEA based on isoprene content for polymer grade isoprene, and essentially all the CPD introduced in the feed stream is then distilled for removal of CPD. The process of this invention makes the sacrifice of isoprene in the overhead unnecessary.
As for a process other than selective hydrogenation for removal of acetylenic impurities from crude isoprene, it will be evident that chemisorption of the impurities on active sites, for later removal of the impurities, necessitates impractically large quantities of adsorbent, even if the adsorbent has high surface area.
It is also known that minute quantities of cyclopentadiene (hereafter "CPD" for brevity), up to about 400 ppm, may be removed from isoprene by selective adsorption of the CPD on supported copper catalysts. Such processes are taught in U.S. Pat. No. 3,492,366 utilizing a fluidized bed copper oxide catalyst in which a hydrocarbon contaminated with CPD is treated at a temperature in the range from about 0.degree. C. to 200.degree. C.; and British Pat. No. 1,125,520, inter alia.
It is noteworthy that, in general a supported copper oxide catalyst disclosed in U.S. Pat. No. 3,492,366 is specifically effective for the removal of CPD in amounts less than 400 ppm when the catalyst is used in a fluidized bed reactor. It has now been found that essentially the same supported cupric oxide catalysts disclosed therein as being so effective for the removal of CPD, are effective in the process of this invention to allow the CPD, if present, to go essentially unconverted. This catalyst was demonstrated in U.S. Pat. No. 3,492,366 to be unexpectedly unable to perform as well in a fixed bed reactor. The peculiar and unexplained preference of a catalyst for CPD adsorption in one reactor over another apparently analogous one, though only the type of bed in the reactor waschanged in this particular process, is further evidence that a particular combination of process conditions is critical when Group I B metal oxide catalysts are used in any practical process for purifying diolefins. This criticality in a practical process is generally reflected and focussed by the absence of examples disclosing mass balances and analyses of the effluent in other relevant prior art disclosures relating to conversion of impurities in the absence of hydrogen.
Polymer grade butadiene for the cis-polybutadiene polymerization system is obtained by purification of butadiene containing unacceptably high levels of apha-acetylenes such as vinyl acetylene and methyl acetylene. U.S. Pat. No. 3,897,511 teaches the selective chemisorption of alpha-acetylenes on copper catalysts activated with NiO, CoO, CrO or MnO. The activated copper catalyst is reduced with hydrogen prior to use. British Patent No. 1,291,397 teaches a mixed CuO/ZnO catalyst which can also be used for chemisorption of alpha-acetylenes.
The prior art is replete with a multiplicity of hydrogenation catalysts particularly suited for hydrogenation of acetylenic and other impurities in conjugated diolefin streams. A few of these catalysts are said to effectively lower the final acetylenic concentration of a feedstream from 1 percent to below 100 ppm without excessive conversion of the diene to a monoolefin or alkane, but the period of time over which the activity can be maintained is quite unpredictable. For one reason or another, some hydrogenation catalysts make for more successful hydrogenation processes than others, and the search for economically competitive processes, whether by hydrogenation or not, continues unremittingly.
Relatively little interest has been directed to the conversion of alkynes by contact with base metal oxide catalysts without hydrogenation or hydration of the alkynes. Catalysts consisting of finely divided copper alone or mixed with an activator metal were known to be useful for removal of the alkynesby selectively decomposing or polymerizing these contaminants, but such a process, inter alia, was known to be subject to one or more disadvantages, as specifically stated in aforementioned U.S. Pat. No. 3,897,511, column 1, lines 26-43. As stated in the earlier Frevel U.S. Pat. No. 2,398,301, temperatures above 200.degree. C. (392.degree. F.) were indicated, 275.degree. C. (527.degree. F.) to 325.degree. C. (617.degree. F.) being preferred (page 2, right hand column, line 38). At these relatively higher temperatures, higher than 300.degree.-360.degree. F., not only alpha-acetylenes but also CPD is removed, and unavoidably, as evidenced by the exothermic reaction noted, a sufficiently large proportion of desirable diolefins are converted to mask the endothermic cracking of acetylenic impurities. We are unaware of any prior art which teaches conversion of acetylenic impurities and their removal by the use of a supported copper oxide or silver oxide hydrogenation catalyst, in the absence of hydrogen, in a predominantly catalytic cracking process.